When pulp is produced in accordance with the sulphate method, a spent liquor, generally termed black liquor, is obtained which contains organic material and the residual chemicals which have been used to cook the fiber raw material. In general, this black liquor is evaporated and conveyed to a separate process for recovering the energy content of the organic material and recovering the cooking chemicals as so-called green liquor. The well-known Tomlinson process has been the commercially dominant method used for this recovery of energy and chemicals. However, a disadvantage of this process, which is now very old, is that it requires very large combustion ovens that are complicated both from the technical point of view and in regards to their operation.
Published Swedish patent application SE 448 173 describes a more recent process that utilizes simplified process equipment and achieves an improved recovery of both energy and chemicals. This process is based on a pyrolysis reaction in which the black liquor is gasified in a reactor, resulting in the formation of an energy-rich gas, principally comprising carbon monoxide, carbon dioxide, methane, hydrogen and hydrogen sulphide, and inorganic chemicals in the form of small drops of smelt, principally comprising sodium carbonate, sodium hydroxide, and sodium sulphide. The resulting mixture of gas and smelt drops is rapidly cooled by means of direct contact with a cooling liquid, usually water and green liquor that is formed when the smelt chemicals dissolve in the cooling liquid. The gas is subsequently washed in a gas-washing apparatus of the scrubber type. The gas is then used as a fuel for generating steam and/or electric power. The physical caloric value of the gas can also be utilized when the gas is cooled down from the gasification temperature to the saturation temperature for aqueous steam at the selected pressure. At a saturation temperature of 252.degree. C., corresponding to 40 bar, for example, steam having a pressure of 3 to 8 bar can be generated when the green liquor is cooled and when the gas is cooled and its water content is condensed downstream of the gas-washing tower.
Nevertheless, this process too, despite being appreciably simpler and smoother than the Tomlinson process, still leaves room for improvement. For example, unwanted carbonate and hydrogen carbonate is formed in the green liquor when carbon dioxide in the pyrolysis gas comes into contact with the green liquor when the gas and smelt droplets are quenched and dissociated in the aqueous solution of the first stage. Therefore, the pH of the green liquor is decreased. It also leads to the undesirable formation of H.sub.2 S which usually must be separated from the desirable gases.
In addition, extremely small, virtually hydrophobic, particles usually remain in the gas when it leaves the gas-washing apparatus because the gas-washing apparatus according to SE 448,173 is not capable of effectively separating them. A further disadvantage is that the recovery of energy from the physical caloric value of the gas cannot be carried out in a manner which is optimal for producing high-quality process steam. Instead, only steam of relatively moderate pressure that can be produced.
Consequently, there is a need for a quench which minimizes the formation of undesirable hydrogen carbonate and is effective at removing small, hydrophobic particles from the combustion gas.